Process for drying bis-({62 -hydroxyethyl) terephthalate

ABSTRACT

Process for removing a volatile liquid medium from solid bis-( Beta -hydroxyethyl) terephthalate wetted with said volatile liquid, by heating said wet bis-( Beta -hydroxyethyl) terephthalate into a molten mass and evaporating said liquid medium. Dried bis-( Beta -hydroxyethyl) terephthalate containing an extremely small amount of volatile liquid can be obtained in a short time without causing degradation in the quality of the product.

United States Patent Ichikawa et al.

[451 June 6, 1972 [54] PROCESS FOR DRYING BIS-(,8-

HYDROXYETHYL) TEREPHTHALATE [72] Inventors: Yataro lchikawa; GentaroYamashita; Mlchiyukl Tokashiki; Nobuo Suzuki, all of lwakuni-shi, Japan[73] Assignee: Teijin Limited, Osaka, Japan [22] Filed: Dec. 24, 1969 21Appl. No.: 888,082

[52] US. Cl. ..260/475 PR, 260/475 P [51] ..C07c 69/82 [58] Field ofSearch ..260/475 PR, 475 R, 475 P [56] References Cited UNITED STATESPATENTS 3,590,070 6/1971 Martin et a1. ..260/475 P 3,600,430 8/1971Martin et al. ..260/475 PR Primary Examiner-Lorraine A. WeinbergerAssistant Examiner-E. Jane Skelly AttorneySherman and Shalloway ABSTRACT5 Claims, No Drawings This invention relates to a process for removingvolatile liquid medium from solid bis-(fl-hydroxyethyl) terephthalatewhich has been wetted with the former, to recover dry bis-(B-hydroxyethyl) terephthalate.

Bis-(B-hydroxyethyl) terephthalate, referred to hereinafter BHET, isproduced in large quantities as an intermediate roduct in themanufacture of polyethylene terephthalate, and has very high industrialvalue. Known methods for the preparation of BHET includeester-interchange between dimethyl terephthalate and ethylene glycol,and direct esterification of terephthalic acid with ethylene glycol.Also recently ethylene oxide process wherein terephthalic acid isdirectly reacted with ethylene oxide has been drawing much attention(for example, refer to Specification of U.S. Pat. No. 3,037,049.) I

Furthermore, it is proposed recently to make BHET by reactingterephthalonitrile and water with ethylene glycol. (This process will behereinafter referred to terephthalonitrile process.) (For example, seespecification of British Pat. No.800,875.) Among those known methods,BHET of less oligomer content can be obtained from ethylene oxideprocess and terephthalonitrile process, particularly ethylene oxideprocess. In the named processes, normally the BHET-forming reaction isconducted in solvent, and the resulting BHET is optionally purified, andseparated from the employed liquid reaction medium or the liquid mediumused for the purification, dried, and sent to the next procedure insolid state.

However, it is extremely difficult to dry such wet, solid BHET,substantially completely eliminating therefrom the solvent used duringthe reaction or the liquid medium used for the purification. Because,when such wet BHET is dried under reduced pressure at relatively lowtemperatures such as 40-60 C., the drying requires objectionably longtime, and quality of BHET is degraded by such phenomenon as coloring,etc. Whereas, in the attempts to dry the solid BHET by heating it torelatively high temperatures, e.g., 6090 C., the surfaces of BHET becomesticky, and the BHET tends to agglomerate into blocks before completionof drying. Thus the operability of drying is impaired and satisfactoryresult of drying becomes difficult to be achieved. I

Accordingly, the object of the present invention is to provide a processfor drying solid BHET wetted with such volatile liquid medium withsimple operation and within relatively short time, without invitingqualitative degradation of the product. I

Another object of the invention is to provide a novel process forrecovering high quality, dry BHET of extremely little volatile liquidmedium content, which is difficult to ob tain through conventionalreduced pressure powder drying methods.

Still many other objects and advantages of this invention will becomeapparent from the following descriptions.

The foregoing objects and advantages are accomplished in accordance withthe subject process for removing volatile liquid medium from solid BHETwhich has been wetted with said volatile liquid medium and recoveringdry BHET, wherein the solid BHET wetted with the volatile liquid mediumis heated and fused into a molten mass, and evaporating from the moltenmass the volatile liquid medium.

Hereinafter the subject process will be explained in further details.

The BHET to which the present drying process can be applied may beprepared by any of the aforementioned methods, while BHET's of lessoligomer contents, such as no more than approximately 20 percent, areparticularly preferred.

Crude BHETs prepared by aforesaid ethylene oxide process andterephthalonitrile process, particularly ethylene oxide process, containrelatively less amounts of oligomer as they are, and therefore aresuited to be directly subjected to the drying process of thisinvention.Obviously, it is also permissible to first purify those crude BHET inthe presence of suitable liquid medium, before applying thereto thesubject drying process.

BHET can also be obtained through depolymerization of ethyleneterephthalate structural unit-containing compounds, such as polyethyleneterephthalate, oligomer of BHET, etc. The process of this invention islikewise applicable for drying such BHET.

The solid bis(B-hydroxyethyl) terephthalate (BHET) wetted with volatileliquid medium" referred to in this specification signifies any BHETwhich is wetted with the volatile liquid medium employed during thepreparation of the BHET as above-mentioned or its purification.

As the volatile liquid media conceived in the invention, for example,the following may be named:

a. Water;

b. Halogenated hydrocarbons such as chloroform, carbon tetrachloride,dichloroethane, trichloroethane, tetrachloroethane, chlorobenzene,bromoform, dibromoethane, bromobenzene, fluorobenzene,a,a',a"-trifluorotoluene, etc.;

c. Ketones such as acetone, methyl ethyl ketone, methyl isopropylketone, diethyl ketone, cyclohexanone, methyl isobutyl ketone,4-methyl-2-pentanone, etc.;

d. Ethers such as diethyl ether, diisopropyl ether, dibutyl ether,tetrahydrofuran, dioxane, etc.;

e. Alcohols such as methanol, ethanol, propanol, isopropanol, butanol,pentanol, ethylene glycol, propylene glycol, etc.;

f. Amides v such as NN-dimethylformamide, NN- dimethylacetarnide,NN-tetramethyleneformamide, NNNN- tetramethylurea,hexamethylphosphonamide, N-alkylpyrrolidone, etc.;

g. Organic acid esters such as methyl acetate, ethyl acetate, butylacetate, methyl propionate, ethyl propionate, butyl propionate, methylphthalate, etc.;

h. Organic cyanides such as acetonitrile, propionitrile, butyronitrile,benzonitrile, succinonitrile, adiponitrile, etc.;

i. Aliphatic hydrocarbons such as pentane, hexane, heptane, octane,nonane, decane, etc.;

j. Alicyclic hydrocarbons such as cyclopentane, methylcyclopentane,cyclohexane, cyclohexene, methyl-, ethyl-, or isopropyl-cyclohexane,cyclooctane, cyclooctadiene, cyclododecatriene, etc.; and

k. Aromatic hydrocarbons such as benzene, toluene, ethylbenzene,isopropylbenzene, o-xylene, m-xylene, p-xylene, pcymene, diethylbenzene,methylnaphthalene, ethylnaphthalene, etc.

Among the named volatile liquid media, particularly those having boilingpoints at atmospheric pressure not lower than 40 C. but below theboiling point of ethylene glycol are conveniently subjected to thepresent drying process. Inter alia, benzene, toluene, and xylene inabove (k) group are preferred liquid media both as the solvents used inthe synthesis of BHET by ethylene oxide process, and as purificationmedia of crude BHET, which furthermore can be very smoothly evaporatedand removed from the solid BHET in the drying process of this invention.Incidentally, use of ethylene glycol or liquid media having boilingpoints higher than that of ethylene glycol is not advantageous, sinceunder the conditions as will evaporate and remove such liquid media,concurrent evaporation of ethylene glycol may take place, enhancing thetendency for oligomerization of BHET. The foregoing liquid media may bepresent in BHET singly or as a mixture of more than one medium.

BHET synthesized and/or purified in the foregoing volatile liquidvolatilizes, medium is normally separated from the excessive medium byknown means of solid-liquid separation, e.g., filtration,centrifugation, etc., but is still wet with the medium after suchseparation. When such wet BHET is, forexample, polycondensed as it is,the ethylene glycol whereby released is taken out of thepolycondensation system together with said medium. Consequently, forlater recovery of the ethylene glycol, it must be separated from themedium mixed therewith, which is quite cumbersome. Again if the wet BHETis stored or transported as it is, the liquid medium contained thereinvolatilizes, which is objectionable both from sanitary and operationalstandpoints. Thus it is necessary to remove the liquid medium from thewet BHET before it is sent to, for example, polycondensation step.

According to the present invention, therefore, the solid BHET wet withsuch volatile liquid medium as aforesaid is heated and fused into amelt, and the volatile liquid medium is evaporated from said melt. Theliquid medium content of the wet Bl-lET to which the subject inventionis applicable is subject to no critical limit, while it is preferredthat solid BHET containing equal amount or less of the liquid medium istreated by the subject process.

As already pointed out, when the wet, solid BHET is heated under areduced pressure to a temperature above the boiling point of the liquidmedium at said pressure, as ordinarily practiced for drying solidpowder, extremely long time is required for removing the liquid mediumand recovering completely dried BHET. Due to such prolonged exposure toheat, furthermore, the quality of BHET is degraded by such phenomenon ascoloring, etc., even at relatively low temperatures such as 4060 C.Again, if the wet, solid BHET is heated to somewhat higher temperatures,for example, 6090 C., the surfaces of solid BHET become sticky, whichinvites not only conspicuous reduction in operability of drying but alsofrequent reduction in drying efficiency.

In clear contrast thereto, when the wet, solid BHET is first heated andfused, and the liquid medium is evaporated from the melt in accordancewith the present invention, the liquid medium can be evaporated andeliminated with high efficiency, within relatively short time.Consequently, according to the drying process of the invention,qualitative degradation of Bl-lET is only little, recovery of the liquidmedium is easy, and high efficiency drying can be performed withapparatus of simple structure.

The reason of very low rate of evaporation of the liquid medium, whenthe wet, solid Bl-lET is heated under a reduced pressure at temperaturesabove the boiling point of the liquid medium at the reduced pressure,is, according to our views, that the liquid medium is adsorbed onto theBHET surfaces due to the unique property of BHET, and therefore it isdifficult to evaporate the adsorbed liquid. Whereas, when the liquidmedium is evaporated from the molten BHET in accordance with theinvention, the evaporation of adsorbed liquid can be very easilyperformed. Thus, the subject process can dry the wet BHET at a rate ofseveral to several hundred times greater than that of conventional,reduced pressure solid drying method.

According to the invention, the wet, solid BHET is heated, and resultingmelt is maintained at temperatures not exceeding 180 C., preferably nothigher than 150 C., while retaining the molten state, from which thevolatile liquid medium being evaporated. The reason why the melt ismaintained at temperatures not exceeding 180 C., particularly not higherthan 150 C., is that, when the melt is maintained at temperatures higherthan the specified temperatures, tendency of coloring or oligomerizationof BHET increases.

The lower limit of the melting temperature of the wet BHET differsdepending on such factors as preparation and/or refining conditions ofthe specific BHET, type and amount of the liquid medium, type and amountof impurities in the BHET, etc., while normally the wet BHET melts attemperatures above 90 C. In special cases, it may be fused at 70-90 C.

As the drying apparatus for conducting the evaporation of liquid medium,for example, conventional agitation type or non-agitation type vessel,film evaporator, flash evaporator, etc. can be used. Provision ofheating device in those evaporators is optional. The drying operationcan be performed either batchwise or continuously, and the wet, solidBHET may be supplied into the drying apparatus in solid form to beheated and fused in the drying apparatus, or in the form of melt whichhas been fused in advance.

The evaporation of liquid medium in accordance with the invention can beperformed within still shorter time and more completely, if the melt ofthe wet, solid BHET is contacted with a flow of inert gas.

The type of the gas is not critical, so far as it is substantially inertto BHET. For example, nitrogen, argon, helium, carbon dioxide, carbonmonoxide, hydrogen, etc. may be used, nitrogen and carbon dioxide,particularly nitrogen, being most preferred from the standpoints ofeconomy and safety of operation. Such inert gas may contain no more thanapproximately 1 percent of molecular oxygen, or mixtures of foregoinggases may be used.

The amount of inert gas to be blown into the system differs depending onthe type and amount of the liquid medium to be removed, dryingtemperature, operating pressure, design of drying apparatus, etc.Whereas, when the vapor pressure of the liquid medium at the dryingtemperature is lower than the operating pressure, the amount of inertgas may be suitably so selected that the average partial pressure of thevaporized liquid medium in the gaseous mixture of the vaporized liquidmedium and the employed inert gas, at the time when all the liquidmedium in the BHET is evaporated, should become lower than the vaporpressure of the medium at the drying temperature. Normally preferredpractice is to use such an amount of inert gas that the partial pressureof the vaporized medium in the gaseous mixture should become 30-85percent of the vapor pressure of same medium at the drying temperature.Such is effective to raise the efficiency of apparatus and increase therecovery ratio of liquid medium. Whereas, when the vapor pressure of themedium to be removed, at the drying temperature, is higher than theoperating pressure, supply of even very minor amount of inert gasremarkably increases the removing efiiciency of the liquid medium.Generally speaking, the preferred amount of inert gas to be blown intothe system to promote the drying effect of the subject process is suchthat the average partial pressure of the vaporized medium in the gaseousmixture composed of the vaporized medium and the inert gas, when thetotal amount of the medium to be removed is evaporated, should becomenot higher than percent of the operating pressure. Also for the purposeof facilitating the recovery of evaporized liquid medium, it isdesirable to so select the supply amount of inert gas, within the abovespecified range, that the partial pressure of the vaporized liquid inthe gaseous mixture should become no less than 20 percent of theoperating pressure.

The melt of wet, solid BHET and inert gas can be contacted by, forexample, blowing the inert gas into the melt, or may be contacted insuch a manner that each of the melt and the inert gas should form acontinuous phase. Or, the melt may be suspended in the current of inertgas. In the first-mentioned blowing system, the blow-in position of theinert gas is not critical so far as it is into the molten BHET, but thatin the vicinity of bottom of the drying apparatus is preferred forincreasing the drying efficiency. For blowing inert gas into the molten,wet BHET, besides the aforementioned drying apparatuses, so-calledPachuca tank apparatus composed of a drying vessel provided with drafttube for inert gas may be employed. Whereas, for contacting the inertgas with the melt, each forming a separate continuous phase, forexample, film evaporator may be used. In that case, the flows of inertgas and molten BHET may be contacted countercurrently or as cocur- Irents, while the latter is preferred for reducing flow rate of the inertgas as well as for increasing the drying efficiency. The inert gas canbe optionally pre-heated.

The drying process of this invention is operable at any of elevated,atmospheric, and reduced pressures, regardless the use of inert gas.Preferred pressure condition is determined according to such factors asthe type and amount of liquid medium contained in the wet, solid BHET tobe dried, and necessity for recovering the medium, etc. Whereas, it isnormally suitable to perfonn the drying, under elevated pressures nothigher than 2 atmospheres as absolute pressure, atmospheric, or reducedpressure.

The greater is the difierence between the boiling point of the liquidmedium to be evaporated and the drying temperature (the latter being thehigher), the shorter may be the drying time. Also the longer the dryingtime, more complete is the drying. However, when the BHET is exposed tosuch drying temperatures as above-described for a prolonged period,polycondensation thereof is induced and its quality, degraded.Therefore, in consideration of thermal stability of BHET, generally thesuitable drying time ranges no longer than hours, particularly no longerthan 3 hours. According to the present invention, it is possible toperform satisfactory drying of the wet BHET within the above preferredtime range.

Thus obtained dry melt of BHET from which the liquid medium has beenremoved may be sent to the next step, e.g., purification step and/orpolycondensation step, as it is, or may be formed into flakes using aflaker, for example, and then sent to such a next step.

The liquid medium evaporated in the drying apparatus may be discarded,or condensed by means of a condenser or scrubber and recirculated forreuse if desired. When an inert gas is used in the drying process ofthis invention, the inert gas may be passed through said condenser,scrubber, or other suitable adsorbing or absorbing apparatus, toseparate therefrom the liquid medium contained therein, and torecirculate the inert gas into the system. The process of this inventionexhibits still additional advantage to those aforedescribed, when aninert gas is thus employed, that the loss in the inert gas is little,and the volatile liquid medium can be recovered at high recovery ratios.

Hereinafter the subject process will be explained in furtherparticulars, referring to the working examples and controls, in whichparts are by weight, unless otherwise specified.

The water content was measured by means of KarlFischers water contentdetermining device. Also the degree of coloring of BHET was measured asfollows: 1.5 g of sample BHET WAS DISSOLVED into pyridine to form asolution of 50 ml, and of which optical density (O.D) measured in a 5-cmcell at a wavelength of 340 mg. was used as the norm of coloring.

EXAMPLE 1 400 parts of crude BHET prepared by reacting terephthalic acidwith ethylene oxide and 1,000 parts of pure water were heated to 100 C.to dissolve the former in the latter. The resulting solution was cooledto 40 C., and the precipitated BHET was separated by filtration, whichwas further washed with water to provide wet, refined BHET. The liquidcontent of the BHET was 32.4 weight percent (based on the wet BHET).

The wet BHET was charged in a glass agitation tank provided with acondenser. The tank was immersed in an oil bath maintained at 130 C.,while purging the air therein with nitrogen gas, to melt the BHET.Maintaining the BHET at the molten state, the steam formed is condensedin the condenser, and withdrawn from the system. The distillation ofwater completed 10 minutesthereafter. Whereupon the molten BHET waswithdrawn, and crystallized by cooling. The water content of the driedBHET was 0.109 weight percent, and its degree of coloring was 0.203 as0D.

CONTROL 1 The wet, refined BHET obtained similarly to Example 1 (watercontent: 32.4 weight percent based on the wet BHET) was dried in avacuum desiccator maintained at 60 C.

minutes were required to reduce the water content to below 10 weightpercent, and the resulting dried BHET had an CD. of 0.233.

' EXAMPLE 2 40 parts of same crude BHET as used in Example 1 was heatedto 50 C. together with 60 parts of refined ethylene glycol, to dissolvethe former in the latter. The resulting solution was cooled to 0 C., andprecipitated BHET was separated, and washed with ice-cooled refinedethylene glycol to provide liquid-containing, recrystallized BHET. Aportion thereof was sampled and dried, to provide a wet BHET containing53.2 weight percent (based on the wet BHET) of the liquid.

The liquid-containing BHET was charged in the same vessel as employed inExample 1. After nitrogen-substitution of inside atmosphere, the vesselwas immersed in a bath of 140 C., to cause fusion of the BHET. When theBHET fused, the pressure within the system was maintained at 50 mm Hg bymeans of a vacuum pump, and evaporated ethylene glycol vapor wascondensed in the condenser and withdrawn from the system.

Distillation of ethylene glycol completed 14.2 minutes thereafter.Whereupon the operation of vacuum pump was stopped and nitrogen gas waspurged with air. The system was cooled, and crystallized BHET waswithdrawn. The ethylene glycol contained in the BHET was extracted withwater, and the quantity of ethylene glycol present in the aqueous phasewas determined by means of gas chromatography, with the result of 0.201weight percent in terms of ethylene glycol content in the dry BHET. TheCD. of dry BHET was 0.21 1.

EXAMPLE 3 Wet BHET prepared by the reaction of terephthalic acid withethylene oxide and following recrystallization of the reaction productfrom methanol (liquid content: 23.4 weight percent, based on the wetBHET) was continuously supplied to the upper part of a drying oven at arate of 2.4 kg/hr. The drying oven was a tank of 1.3-liter capacity, andheating was effected by the jacket and the coil within the tank. TheBHET in the tank was maintained at molten state at C. On the upper partof the tank, a heat exchanger was provided at such a location as wouldcause no refluxing of condensed methanol into the tank. Nitrogen gas wasconstantly passed through the tank, and the operation was run underatmospheric pressure. As soon as the liquid-containing BHET was fed intothe drying oven, the methanol therein was flash evaporated, and theBHET, melted. The vaporized methanol was condensed in the heat exchangerand recovered.

Thus dried molten BHET was continuously withdrawn from the lower part ofthe drying oven at a rate of 2 liters/hr., and sent to a flaker.

Methanol in thus obtained BHET was extracted with water and quantitythereof was determined by means of gas chromatography. When converted todry BHET basis, the methanol content of the dry BHET was 0.023 weightpercent. The dry BHET had an OD. ofO.20l.

EXAMPLE 4 30 parts of crude BHET prepared by reacting terephthalic acidwith ethylene oxide and 70 parts of pure water were heated to 100 C. todissolve the former in the latter. The resulting solution was cooled toroom temperature, and the precipitated BHET was separated by filtration,and washed furtherwith water to provide wet, refined BHET of a watercontent 32.4 weight percent (based on the wet BHET).

The wet BHET was charged into a glass agitation tank which was providedwith a gas condenser and a gas inlet nozzle at the bottom, andmaintained at substantially atmospheric pressure, and immersed in an oilbath of C. The BHET was whereupon melted. While maintaining the BHET atthe molten state, nitrogen was supplied into the tank from the bottom ata fixed flow rate, and the steam evaporated was condensed in thecondenser and withdrawn from the system. The inert gas was sent into awet type gas meter to be measured of its integrated flux, and dischargedinto air. 10 minutes after the operation started, the molten BHET waswithdrawn, cooled and crystallized. The water content of the crystallineBHET was 0.018 weight percent. The average partial pressure of steam inthe gaseous mixture discharged from the tank, as calculated from thedistilled water and integrated flux of inert gas, was 0.34 atm. Also thedegree of coloring of dry BHET in terms of the CD. was 0.20.

40 parts of the same crude BHET as employed in Example 1 was heated to50 C. together with 60 parts of refined ethylene glycol, to dissolve theformer in the latter. The resulting solution was cooled to C., and theprecipitated BHET was separated, and washed with ice-cooled, refinedethylene glycol to provide liquid-containing, recrystallized BHET. Aportion thereof was sampled, dried, and measured of the liquid content,which was 53.2 weight percent (based on the wet BHET).

The wet Bl-lET was charged into the same vessel as employed inExample 1. After nitrogen substitution of the inside atmosphere, thevessel was immersed in a bath of 140 C. to cause fusion of the BHET.Upon melting of the BHET, inert gas below-specified was continuouslysupplied into the vessel through the inlet nozzle at the bottom thereof,while maintaining the inside pressure of the system at 60 mm Hg by meansof a vacuum pump, and the vapor of evaporated ethylene glycol wascondensed in the condenser and withdrawn from the system.

The vacuum pump was stopped 14.2 minutes thereafter, and the used inertgas was purged with air to eliminate the vacuum. Cooled Bl-lET was takenout, and ethylene glycol contained therein was extracted with water. Theethylene glycol in the aqueous phase was determined by means of gaschromatography. The ethylene glycol contents calculated on dry BHETbasis and the OD. values of the dry BHET are shown in the table below.

100 parts of BHET containing 54.2 weight percent (wet BHET basis) ofbenzene which was obtained by reacting terephthalic acid with ethyleneoxide in benzene as the solvent, was charged in the same vessel asemployed in Example 1, and melted in an oil bath maintained at 120 C. tobe fused.

When the system was maintained at 120 C. over a period of 25 minutes,the distillation of benzene stopped. Then, the content of the reactorwas taken out and the benzene content was determined by gaschromatography. The results are shown in table below.

Next, the above benzene-containing wet BHET was charged in a reactorsame as used above, and melted on an oil bath.

While maintaining the BHET in molten state, nitrogen was fed into thevessel from the bottom thereof at a fixed flow rate under atmosphericpressure, and the evaporated benzene vapor was condensed in thecondenser and withdrawn out of the system. The inert gas was sent into awet type gas meter to be measured of the integrated flux, and thendischarged into air. minutes after the nitrogen supply started, thesupply was stopped and the molten BHET was withdrawn, cooled andcrystallized. The benzene content of the crystalline BHET wasdetermined. The results are shown in the table below. The averagepartial pressure of the benzene vapor in the discharged gaseous mixtureas calculated from the amount of benzene obtained by condensation in thecondenser and the integrated flux of nitrogen gas was 0.37 atm.

A toluene-containing wet BHET synthesized with the use of toluene assolvent (toluene content being 61.5 percent by weight) was treated at130 C. in the same manner as above. The toluene content of the resultingdry BHET is also shown in the table below.

Medium content Class of in dry 13H ET Example reaction medium N, gasblowing by weight) 6a None 0.100

Benzene 6b Yes 0.0091 6-c None 0.302

, Toluene d Yes 0.105

CONTROLS 2-5 Control Type of Reaction Liquid Content No. Medium of DryBHET 2 Benzene 5.3 2 3 Toluene 8.01

When the wet Bl-lETs of Example 6 were each put in a 40 C. vacuumdesiccator and withdrawn 5 hours thereafter, their respective benzeneand toluene contents were as given in the table below.

Control Type of Reaction Liquid Content No. Medium of Dry BHET 4 Benzene5.21 5 Toluene 6. 3 3

EXAMPLE 7 In an extraction apparatus composed of a recovery section(three mixer-settlers), a purification section (eight mixer-settlers), acooling condenser and a reflux box, the recovery and purificationsections were maintained at 105 C., and the reflux box, at C. From theend of recovery section, 100 parts per unit time of pro-heated toluenewas continuously supplied, and from the other end of the recoverysection, molten BHET obtained by reacting crude terephthalic acid withethylene oxide was continuously supplied at a rate of 7.5 parts per unittime. The toluene solution of Bl-lET at C., as recovered at thepurification section, was cooled to 100 C. in the cooling condenser,whereby separating and settling a part of molten BHET from the solution.The melt phase separated as a heavy fluid phase at the reflux box wasreturned to the purification section and refluxed. Thus the toluenephase and molten BHET phase were continuously contacted countercurrentlyin the purification section. The toluene phase separated from the moltenBHET phase at the reflux box was continuously withdrawn from the system,and allowed to cool off to 20 C. to cause precipitation of BHET,followed by solidliquid separation. Thus obtained cake was washed withcold toluene, and whereby refined cake contained 5.23 weight percent oftoluene.

This wet BHET was molten at C., and continuously fed into the dryingagitation vessel employed in Example 1 which was maintained at 120 C.,at such a rate that its average staying time in the vessel should becomean hour. The vessel was maintained at substantially atmosphericpressure, and nitrogen gas was continuously blown thereinto from thebottom. The content of the vessel was continuously withdrawn to maintainthe liquid level in the agitation tank constant. Thus obtained BHETcontained 0.136 weight percent of toluene, and had an CD. of 0.021.

The average partial pressure of the toluene vapor in the dischargedgaseous mixture as calculated from the benzene obtained during thedrying operation by condensation and the integrated flux of nitrogen gaswas 0.42 atm.

This BHET was added with 0.030 mol percent of antimony trioxide to theBHET. After nitrogen substitution of inside atmosphere of the vesselcontaining the BHET, the vessel was immersed in a bath of 285 C. andmaintained at atmospheric pressure for 30 minutes. Thereafter thepressure was gradually reduced to 1 mm Hg or somewhat less over thesubsequent 30 minutes, followed by 60 minutes polymerization under saidcondition. Thus, two hours after the immersion in the bath, the vacuumwas broken with nitrogen gas, and the system was cooled to providepolymer. Thus obtained polymer exhibited excellent quality asdemonstrated by the following data:

[1;]:0.653 S.P.:262.4Color tone: L:84.3 a:-0.5 b:3.6

[COOl-l]: 14.8 eq./l g.

In the above, [1;] is the intrinsic viscosity of the product polyestermeasured at 35 C. in orthochlorophenol as the solvent; S.P. is thesoftening point of the polyester; and L", a, and b of color tone are thereadings on color-difference meter in accordance with the indicationtable of ASTM 1482-57T.

EXAMPLES 8-15 15 parts of crude BHET'synthesized from terephthalic acidand ethylene oxide and 85 parts of below-specified liquid medium wereboiled at the boiling point of said medium, to cause completedissolution of the BHET. If the BHET could not be completely dissolved,more medium was added. The system was then cooled to room temperature,and the precipitated BHET was separated. The liquid content of thusobtained wet BHET (on wet BHET basis) in each run was as shown in thetable below.

Then the wet BHET was charged in the identical vessel with that employedin Example 1, immersed in an oil bath, and heated to 120 C. atatmospheric pressure to cause melting of the BHET.

Nitrogen gas was fed into the vessel from the bottom, at such a ratiothat the average vapor pressure of the medium in the discharged gasshould become 0.3 atm. 15 minutes thereafter the molten BHET waswithdrawn, cooled and crystallized, and the product's liquid content wasdetermined by means of gas chromatography, with the result shown in thetable below. In said table, the run using xylene as the liquid mediumwas run at a reduced pressure of 100 mm Hg.

Liquid content EXAMPLES 16-21,

An autoclave was charged with 83 parts of crude terephthalic acid, 430parts of benzene, 44 parts of ethylene oxide, and 0.3 part oftriethylamine, and after nitrogen substitution of its inside atmosphere,nitrogen was further introduced to a pressure of kglcm G. The contentwas heated BHET was each measured of its benzene content and OD,

with the results given in the same table.

Benzene Temp. for Maintenance content maintaining time of of dryOligomcr molten state BHET at BHET O.T). of content of Ex. oi BHETmolten state (wt. dry dry BHF'T No. 0.) (min.) percent) BHET (wt.percent) The oligomer content of BHET was determined by means of liquidchromatography of the solution of BHET in dimethylformamide solvent. Theoligomer content of the starting crude BHET was 1.02 percent.

EXAMPLE 22 An autoclave was charged with 100 parts of terephthalonitrilesynthesized by vapor phase air oxidation of p-xylene in the presence ofammonia, 4,800 parts of ethylene glycol, 28 parts of water, 0.3 part ofzinc acetate, and 0.3 part of trimethyl phosphate. Nitrogen gas was fedinto the autoclave, and while driving the released ammonia gas outsidethe system, the system was reacted for 7 hours at 210 C. The resultingreaction mixture was cooled to room temperature, and then separated intocrude BHET and ethylene glycol. The separation however was ratherdifficult, and the crude BHET contained 80.1 weight percent of theliquid (wet BHET basis).

This wet BHET was charged in the same vessel as employed in Example 1,and subjected to the identical drying procedure with that described inExample 2 at 130 C. Distillation of ethylene glycol completed within18.5 minutes, and whereupon the molten BHET was withdrawn and cooled.The ethylene glycol remaining in the product was extracted with waterand determined by means of gas chromatography, which was 0.31 weightpercent.

7 EXAMPLE 23 Polyester waste yarn was washed with soapsuds, thoroughlyrinsed with water, and dried. A vessel equipped with a reflux condenserwas charged with 100 parts of thus dried waste yarn, together with 2,000parts of ethylene glycol, and the content was boiled at the boilingpoint of ethylene glycol at atmospheric pressure for 3 hours. Thereafterthe insoluble matter was hot-filtered, and remaining ethylene glycolsolution containing BHET was cooled to 40 C. Thus separated precipitatedBHET contained 48.3 weight percent of the liquid.

This wet BHET was dried in the identical manner with Example 2, exceptthat the nitrogen gas was charged into the vessel from a lower partthereof, at such a rate to make the partial pressure of ethylene glycolvapor in the discharged gaseous mixture 0.4 atm.

The ethylene glycol content of thus obtained dry BHET was 0.075 weightpercent.

EXAMPLES 24-25 A slurry composed of 83 parts of terephthalic acid, 44parts of ethylene oxide, 288 parts of benzene, and 0.5 part oftriethylamine as the catalyst was continuously fed into a tubularreactor of which temperature was controlled to be 180 C.,

at a rate of 10.56 parts per minute through a plunger pump. The tubularreactor was made of stainless steel pipe of 4 mm in diameter and 140 min length, which was immersed in cylindrical water tank. The averagestaying time of the reaction liquid was minutes.

The reaction mixture was taken out into a tank maintained at 1 10 C. and1.4 kg/cm G through an intermediate tank of the pressure controlled tobe 20 kglcm G. During that procedure, the evaporated benzene waswithdrawn from the system and recovered. Subsequently, the reactionmixture was continuously sent to a pressure filter to be removed ofunreacted terephthalic acid, and wherefrom continuously sent to aprecipitation tank maintained at the boiling point of benzene atatmospheric pressure. Thus precipitated BHET slurry was sent to acentrifugal machine to effect solid-liquid separation. Thus wet BHETcontaining 23.5 weight percent of the liquid was obtained, which wascontinuously bed into a melting tank at a rate of 13 kg/hr. The meltingtank was a 50-liter capacity vessel equipped with a steam-heatingjacket, heating coil at inside, and a guide cylinder at the center, thusforming a Pachuca tank. The inside temperature thereof was maintained at120 C. at atmospheric pressure.

From the bottom of said vessel, nitrogen gas was supplied at a ratiospecified in the table below for each run, and the gaseous mixture ofnitrogen and benzene discharged from the upper part of the vessel wasfirst led out of the system, and separated into benzene and nitrogen bymeans of a scrubber. The nitrogen gas was recirculated into the meltingtank through a blower.

The average staying time of the BHET in the tank was 1 hour. The moltenBHET was continuously withdrawn from the tank bottom, sent to a flaker,cold-molded, and recovered.

The benzene content and CD. of the dry BHET in each run were as follows:

The wet BHET prepared in Example 23 was first melted by heating, andcontinuously supplied to upper part of a film evaporator manufactured byLuwa Co. The evaporator was a vertical cylinder equipped with asteam-heating jacket, maintained at C. Nitrogen gas was supplied from alower part thereof at such a rate that partial pressure of benzene vaporin the discharged gaseous mixture should become 0.4 atm. The insidepressure of the evaporator was controlled to be 100 mm Hg by means of aradiator. The evaporated benzene was led out of the system, condensed bya guard condenser, and recovered. The molten BHET withdrawn from a lowerpart of the evaporator was sent to a fiaker to be cold-molded.

The resulting flaky BHET contained 0.0032 weight percent of benzene.

EXAMPLE 27 The recrystallization filtrate of 40 C. obtained byrecrystallizing the crude BHET from water IN Example 1 was furthercooled to 5 C. Whereby 20 parts of precipitated BHET was recovered. Thiswet BHET (liquid content: 35.2 weight percent was molten and dried inthe identical manner with Example l. The water content of the resultingdry BHET was 0.156 weight percent, and its 0D. was 0.236. terephthalate.

We claim: l. A process for removing a volatile liquid medium from solidbis-(B-hydroxyethyl) terephthalate wet with said volatile liquid mediumand recovering dry bis-(B-hydroxyethyl) terephthalate, which comprisesheating and fusing solid bis- (B-hydroxyethyl) terephthalate wet with avolatile liquid medium having a boiling point at normal pressure notlower than 40 C. but lower than the boiling point of ethylene glycolinto a molten mass, and evaporating said volatile liquid medium from themelt without causing degradation of said bis-(flhydroxyethyl)terephthalate.

2. The process of claim 1, wherein said volatile liquid medium isevaporated from the melt of wet, solid bis-(B-hydroxyethyl)terephthalate which is maintained at the molten state at temperaturesnot higher than C.

3. The process of claim 1 wherein said volatile liquid medium isselected from the group consisting of benzene, toluene, and xylene.

4. The process of claim 1 wherein said bis-(B-hydroxyethyl)terephthalate is that obtained by reaction of terephthalic acid withethylene oxide.

5. The process of claim 1 wherein said melt is contacted with a flow ofinert gas.

2. The process of claim 1, wherein said volatile liquid medium isevaporated from the melt of wet, solid bis-( Beta -hydroxyethyl)terephthalate which is maintained at the molten state at temperaturesnot higher than 180* C.
 3. The process of claim 1 wherein said volatileliquid medium is selected from the group consisting of benzene, toluene,and xylene.
 4. The process of claim 1 wherein said bis-( Beta-hydroxyethyl) terephthalate is that obtained by reaction ofterephthalic acid with ethylene oxide.
 5. The process of claim 1 whereinsaid melt is contacted with a flow of inert gas.